The amulet should be found in one of the major planned dungeons. Wearing the said amulet removes or reduces the fog in the dimension for the wearer, likely through a mixin to the fog rendering code.

Now that we have more control over fog rendering, maybe this idea could be implemented?

It could, although Pyrofab is working on our own custom fog it seems, which is why I haven't bothered.

Expanding on this (and repeating a discussion in discord), it may be best to write our own fog renderer. The vanilla one (which is just a call to the GL fog renderer) doesn't look good. If we did this, we could allow each biome to define fog color & intensity, which would make it easier to implement "if the player is wearing this ring, don't render the fog" kind of logic.